Printed circuit board mesh routing to reduce solder ball joint failure during reflow

ABSTRACT

Voids are introduced in a copper shape to reduce warpage experienced by a printed circuit board during a reflow process. Copper shapes on an outer layer of a printed circuit board may be used to connect large packages that include ball grid arrays to the printed circuit board. The copper shapes may induce warpage in the printed circuit board during the reflow process. Routing a mesh pattern of voids in the copper shapes may reduce solder ball joint cracking and pad cratering during reflow and make solder joints more reliable. The voids may make the copper shapes less ridged and change the copper heat dissipation profile to remove sharp warpage forces that cause solder joints to experience pad cratering. The voids may be 8 mil x 8 mil cuts or indentations in the copper shape.

BACKGROUND

A printed circuit board may support and connect electronic components. Aprinted circuit board may include a flat sheet of insulating materialand a layer of copper foil laminated to the substrate. Chemical etchingmay divide the copper into separate conducting lines, pads forconnections, vias to pass connections between layers of copper, andfeatures such as solid conductive areas for electromagnetic shielding.The tracks may function as wires fixed in place that are insulated byair and the board substrate material.

Components may be soldered onto the printed circuit board. One way toattach components to the printed circuit board is through surfacemounting. “Surface mount” components may have leads or other featuresthat are attached to copper traces or pads on the printed circuit board.A surface-mount component may have short pins or leads of variousstyles, flat contacts, a ball grid array, or terminations on the body ofthe component. A ball grid array may be a type of surface-mountpackaging used for integrated circuits. A ball grid array may consist ofan array of solder balls that are attached to a bottom side of a chippackage.

After components are placed on the printed circuit board, the printedcircuit board may be placed into a reflow soldering oven to undergo asolder reflow process. The solder reflow process may bond the ball gridarray to pads on the printed circuit board. During the reflow process,the printed circuit board may experience warpage. Warpage may lead topad cratering, which impacts reliability of the printed circuit board.

SUMMARY

In accordance with one aspect of the present disclosure, a system isdisclosed for mitigating warpage of a printed circuit board. The systemincludes the printed circuit board. The printed circuit board includesan outer copper layer and a copper shape within the outer copper layer.The copper shape has an outer edge that encloses an area and the area ofthe copper shape includes voids. The printed circuit board also includesan array of pads within the area enclosed by the copper shape. Thesystem also includes a package. The package includes at least onesilicon die and a ball grid array. The ball grid array includes solderballs configured to connect with the array of pads.

The voids may be sufficient in number to reduce a difference indisplacement experienced during a reflow process between a middle of thecopper shape and an edge of the copper shape.

The voids may be arranged in a mesh pattern and the mesh pattern may bearranged over 10% of the area enclosed by the copper shape.

The voids may be 8 mil × 8 mil.

The at least one silicon die may include two or more silicon die.

The solder balls may not be uniform in height.

The printed circuit board may include three or more layers of copper.

The voids may not penetrate to a layer of copper below the outer copperlayer.

A region of the area enclosed by the copper shape may not include thevoids.

The array of pads may include one or more pads connected directly to thecopper shape. The copper shape may provide power to the package. Theregion may be more proximate to the one or more pads connected directlyto the copper shape than at least one other region of the copper shape.

The copper shape may include 30 or more voids.

One or more of the voids may be empty.

In accordance with another aspect of the present disclosure, a printedcircuit board is disclosed that includes an outer copper layer and acopper shape within the outer copper layer. The copper shape includes atleast 10 voids. The printed circuit board also includes an array of padsarranged within an area of the copper shape. The array of pads areconfigured to receive a ball grid array attached to a package.

The voids may be sufficient in number to reduce warpage experienced bythe copper shape during a reflow process.

The voids may be arranged in a mesh pattern.

One or more of the voids may be at least 8 mil × 8 mil.

The printed circuit board may include three or more layers of copper.

The voids may not penetrate to a layer of copper below the outer copperlayer.

The copper shape may include voids arranged over at least fifty percentof a portion of the copper shape that does not include the area wherethe array of pads is arranged.

In accordance with another aspect of the present disclosure, a method isdisclosed for mitigating warpage in a printed circuit board. The methodincludes fabricating a copper shape on a top layer of the printedcircuit board, fabricating an array of pads within an area of the coppershape, and introducing voids in the copper shape over a region outsidethe area. The region includes at least ten percent of the copper shape.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

Additional features and advantages will be set forth in the descriptionthat follows. Features and advantages of the disclosure may be realizedand obtained by means of the systems and methods that are particularlypointed out in the appended claims. Features of the present disclosurewill become more fully apparent from the following description andappended claims, or may be learned by the practice of the disclosedsubject matter as set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to describe the manner in which the above-recited and otherfeatures of the disclosure can be obtained, a more particulardescription will be rendered by reference to specific embodimentsthereof which are illustrated in the appended drawings. For betterunderstanding, the like elements have been designated by like referencenumbers throughout the various accompanying figures. Understanding thatthe drawings depict some example embodiments, the embodiments will bedescribed and explained with additional specificity and detail throughthe use of the accompanying drawings in which:

FIG. 1A illustrates an example copper shape with a mesh pattern ofvoids.

FIG. 1B illustrates an example printed circuit board that includes acopper shape with a mesh pattern of voids.

FIG. 2 illustrates an example copper shape that does not include voids.

FIGS. 3A-3C illustrates an example pair of a ball grid array attached toa package and an array of pads within a copper shape that includes amesh pattern of voids.

FIG. 4 illustrates improvement printed circuit board displacementresulting from introducing voids in a copper shape.

FIG. 5A illustrates areas of a copper shape that may be designated forpads and voids.

FIG. 5B illustrates example patterns for voids in a copper shape.

FIG. 6 illustrates an example method for mitigating warpage in a printedcircuit board during a reflow process.

DETAILED DESCRIPTION

This disclosure concerns use of a mesh pattern of voids etched in acopper shape. The copper shape resides on a top layer of a printedcircuit board underneath a large package (such as a multi-die NAND). Themesh pattern of voids reduces warpage experienced by the printed circuitboard and the copper shape during a reflow process. The reflow processbonds solder balls attached to the large package to corresponding padson the printed circuit board. Reducing the warpage of the printedcircuit board during the reflow process mitigates the risk of the padsseparating from the printed circuit board during reflow and makesconnections between the large package and the printed circuit board morereliable long term.

A printed circuit board (PCB) may support and connect electrical orelectronic components. A PCB may include a flat sheet of insulatingmaterial (a substrate) and a layer of copper foil laminated to thesubstrate. Chemical etching may divide the copper into separateconducting lines (called tracks or circuit traces), pads for connectingcomponents to the PCB, vias to pass connections between layers ofcopper, and features such as solid conductive areas for electromagneticshielding or other purposes. The tracks may function as wires fixed inplace that are insulated by air and the board substrate material.

PCBs may be single-sided (one copper layer), double-sided (two copperlayers on both sides of one substrate layer), or multi-layer (outer andinner layers of copper, alternating with layers of substrate). Formulti-layer PCBs, the copper of each layer may be separated intoconducting lines, pads, vias, and features insulated from each other andother copper layers by substrate. Stripline may be a transmission linetrace surrounded by dielectric material suspended between two groundplanes on internal layers of a PCB. Microstrip routing may be atransmission line trace routed on an external layer of the board.Microstrip may be separated from a single ground plane by a dielectricmaterial. Multi-layer PCBs may allow for higher component density bymoving circuit traces that would otherwise use space on a top layer toan inner layer. Conductors on different layers may be connected withvias, copper-plated holes that function as electrical tunnels throughthe insulating substrate. Through-hole component leads may alsoeffectively function as vias. In four-layer designs, often two layersare dedicated as power supply and ground planes, and the other two areused for signal wiring between components.

Components may be soldered onto the PCB to both electrically connect andmechanically fasten the components to the PCB. Solder may be anelectrical material that can bond parts. There may be at least two waysto attach components to the PCB. “Through hole” components may have wireleads and may be mounted by their wire leads passing through the boardand soldered to traces on the other side. “Surface mount” components mayhave leads or other features that are attached to copper traces or padson a surface layer of the board. A surface-mount component may besmaller than a through-hole counterpart because the surface-mountcomponent may have smaller leads or no leads at all. A surface-mountcomponent may have short pins or leads of various styles, flat contacts,a matrix of solder balls (such as a ball grid array), or terminations onthe body of the component. A PCB may use both methods for mountingcomponents.

A ball grid array (BGA) may be used as part of surface-mount packaging(a chip carrier) for integrated circuits. A BGA package may be used topermanently mount devices, such as microprocessors, to a PCB. A BGApackage may consist of an array of solder balls that are attached to abottom side of the package body or carrier of a component. A solder ball(which may also be called a solder bump) may be a ball of solder thatprovides a contact between a chip package and a PCB. The solder ballsmay be placed manually or by automated equipment and may be held inplace with a tacky flux. Heights of the solder balls may vary across aBGA. For example, solder balls on an edge of the BGA may be slightlyshorter than solder balls in the middle of the BGA.

A PCB may have flat copper pads (which may be tin-lead, silver, or goldplated) where components are to be placed. These pads may be referred toas solder pads. Solder paste, which may be a sticky mixture of flux andtiny solder particles, may be applied to the solder pads using a screenprinting process. A stainless steel or nickel stencil (which may bereferred to as a mask) may include open windows where the pads arelocated and mask everything else. The stencil may be placed over the PCBand a squeegee may be drawn across the stencil to apply solder paste tothe pads where components may be attached. The solder paste may also beapplied with a jet-printing mechanism, similar to an inkjet printer.After pasting, the PCB may proceed to a pick-and-place machine thatplaces components on the PCB. The paste may temporarily hold thecomponents in place on the PCB. The paste may not, however, permanentlykeep the components in place.

After the pick-and-place machine places components on the PCB, the PCBmay be placed into a reflow soldering oven to undergo a solder reflowprocess (which may be referred to as a reflow process) that bonds thecomponents to the PCB. The solder reflow process may be a controlledtemperature process that includes multiple stages, such as temperatureramp, constant temperature soak, and cooling. For example, the PCB mayramp up to a pre-heat zone. In the pre-heat zone, the temperature of thePCB and all the components may be gradually and uniformly raised. ThePCB may then ramp up to a temperature zone in which the temperature ishigh enough to melt the solder particles in the solder paste, bondingthe component leads to the pads on the PCB. The surface tension of themolten solder may help keep the components in place. After soldering,the PCB may be washed to remove flux residues and any stray solder ballsthat could short out closely spaced component leads. The PCB may bevisually inspected for missing or misaligned components and solderbridging and tested for correct operation.

Connections between components and a PCB need to be reliable. Long-termreliability of the bond created through the reflow process is criticalto ensure devices do not fail in the field over time. Large components,such as a controller, a DRAM, or a NAND, may have many attach points toa PCB. These large components may have a number of solder balls or use aBGA to connect to a number of landing spots (or pads) on the PCB. Thesolder balls may be part of the component packaging and then connect tosolder paste on the PCB pads during the pick and place process. Thesolder balls may bond to the pads during the reflow process.

The reliability of connections between components and the PCB may betested using a technique called dye and pry. The first step of the dyeand pry process may include submerging the PCB in red dye and leaving itto soak for an extended period of time. The time allows the dye to gounderneath components and packages attached to the PCB and into allvoids, crevices, and cracks between bonding sites. After the PCB hassoaked in the red dye, the PCB may be baked to remove all bubbles. Inthe next stage of the process, a tool mechanically rips components offthe PCB. The PCB and the components may be inspected using a microscopeto see if bonds are completely clear of dye. The dye may highlightcracks created in solder joint connections during the bonding process.

The dye and pry technique may reveal multiple different types offailures. A first type of failure may be a component pad separating froma component substrate. A second type of failure may be a separationbetween the component pad and a solder ball. These first two types oferrors may be errors caused by a component manufacturer. A third type offailure may be a separation between a solder ball and a PCB pad. Afourth type of failure may be a separation between a PCB pad and a PCBbase. A fifth type of failure may be a solder ball crack or doubleseparation.

A dye and pry test may reveal pad cratering, which may be an example ofthe fourth type of failure. Pad cratering may refer to a weak jointbetween the PCB pad and the PCB laminations below the pad. Pad crateringmay refer to cracks being generated and penetrated into the PCB itself.The cracks may appear between the PCB pad and the substrate of the PCB.Pad cratering may occur at a solder joint (where a solder ball connectsto a PCB pad). Pad cratering may result when the PCB pad (which may beconnected to a solder ball) pulls off from the PCB lamination materialsduring the reflow process.

Pad cratering may result from the PCB warping during the reflow process.The PCB may itself warp during the reflow process. The package may alsocause or contribute to the warping. Some packages include multiple dieinside the package. For example, certain NAND solutions may includemultiple die stacked one top of each other inside a package. Dual dieper package (DDP) means a stack of 2 die in the same package, quad dieper package (QDP) means a stack of 4 die in the same package. Placingpackages with multiple die inside may exacerbate warping. As the numberof die stacked in a package increases, the package may induce greaterwarpage stress on the PCB during the reflow process.

During the solder cool down phase, the warpage stress from a package maybe focused on certain solder ball zones of the package and may lead tosolder ball cracking. As noted previously, solder balls on an edge ofthe BGA may be slightly shorter than solder balls in the middle of theBGA. Solder balls at the edge of a package may show greater compressionwhile solder balls in the middle of the package may show greaterpulling. The solder balls in the middle of the package may experiencetremendous stress during the reflow package. Warpage forces that pull onsolder joints may lead to pad cratering.

In addition to warpage forces from the package, large copper shapesunder the package may impact heat dissipation during the reflow processand cause warpage forces to focus on specific solder joints duringreflow. A large package, such as a NAND package, may be placed on asurface layer of the PCB that has a large copper shape for powerdelivery. A Shadow Moiré test performed on a PCB with a large coppershape may show large warpage forces focus on specific pads. A ShadowMoiré test may be a test performed on a PCB that does not yet havecomponents placed on the PC. The Shadow Moiré test may measure warpageacross a PCB or a copper shape at different temperature zones.

The most significant factor in pad cratering may not be the magnitude ofdisplacement experienced at any particular solder joint. Instead, padcratering may be most severe at solder ball joints located at inflectionpoints. An inflection point may be a location where deformationdirection is changing from suppression to pulling. The inflection pointmay also be a point with the highest rate of change in deflection. Asolder joint may be most susceptible to pad cratering if the solderjoint is located between a first solder ball that is shorter andexperiencing depression during the reflow process and a second solderball that is taller and experiencing pulling during the reflow process.Thus, pad cratering may be most severe where the solder ball heights arechanging most rapidly.

Large component packages require power. As noted above, one way a PCBmay bring power to a package is through a copper shape on a top layer ofthe PCB. A copper shape may be a large piece of copper. The copper shapemay be larger than other pads used to connect components to the PCB. Thepackage may be placed over at least a portion of a copper shape andattached to at least one pad that directly connects to or is part of thecopper shape. The copper shape may be a large, ridged piece of copper.During the reflow process, the heat dissipation of the copper shape maylead to warpage.

The present disclosure concerns systems and methods for making solderjoints more reliable long term. A PCB mesh routing technique may reducesolder ball joint cracking and pad cratering during reflow. The PCB meshrouting technique may reduce solder ball joint cracking and padcratering caused by warpage forces from large, multi-chip packagesplaced on a copper shape of a PCB. The mesh routing may take the form ofsmall voids introduced in the copper shape. The small voids may make thecopper shape less ridged and reduce warpage. These small voids maychange the copper heat dissipation profile to remove the sharp warpageforces that cause solder joints to experience pad cratering. The smallvoids may be 8 mil × 8 mil cuts or indentations in the copper shape butdo not go through to another copper layer of the PCB. The voids may beadded in a mesh pattern. Even with the voids, the copper shape maydeliver sufficient power to the package. A PCB that includes a coppershape with mesh routing may pass a dye and pry test.

FIG. 1A illustrates an example copper shape 102 that includes voids 100.The copper shape 102 may be a piece of copper etched on a top layer of aPCB. The copper shape 102 may be part of the top layer of copper on thePCB. In FIG. 1A, everything within an outer boundary of the copper shape102 not shown in dark color, subject to two exceptions described below,may be copper. The copper shape 102 may be larger in surface area thanone or more other pads residing on the top layer of the PCB. The coppershape 102 may have a largest surface area compared to all other pads orcopper features residing on the top layer of the PCB. The copper shape102 may be configured to connect with a package containing an integratedcircuit. The copper shape 102 may be ridged. The copper shape 102 shownin FIG. 1A has a generally rectangular shape. But the copper shape 102may take the shape of any polygon. For example, the copper shape 102 maybe in the general shape of a circle, a triangle, a square, a trapezoid,a parallelogram, a pentagon, or a hexagon. The copper shape 102 may alsohave an irregular shape. As shown in FIG. 1A, the copper shape 102 mayinclude protrusions 110 a, 110 b.

Within an area encompassed by a boundary (or outside edge) of the coppershape 102, there may be pads 106 a and pads 106 b. The pads 106 a mayinclude pad 108 a, and the pads 106 b may include pad 108 b. The pads106 a, 106 b may be copper pads arranged in a pattern. The pads 106 a,106 b may be on the top layer of the PCB. Except for the pad 108 a andthe pad 108 b, the pads 106 a, 106 b may be surrounded by anti-pad. Forexample, the pads 106 a may include pad 106 a-1, which may be surroundedby anti-pad. The anti-pad may insulate the pads 106 a, 106 b from thecopper shape 102. Except for the pad 108 a and the pad 108 b, the pads106 a, 106 b may connect to transmission lines in an inner layer or abottom layer of the PCB. The pad 108 a and the pad 108 b may be part ofthe copper shape 102 or connect directly to the copper shape 102. Thecopper shape 102 may provide power to a package attached to the pads 106a, 106 b through the pad 108 a and the pad 108 b. Although FIG. 1A showstwo pads, the pads 108 a, 108 b, for providing power to the package, inother designs, the pads 106 a, 106 b may include fewer than or more thantwo pads that provide power to a package attached to the pads 106 a, 106b.

The pattern of the pads 106 a, 106 b may correspond to a pattern ofsolder balls of a BGA. The BGA may be part of a package. The package maybe placed on the pads 106 a, 106 b. A large portion of the copper shape102 may reside beneath the package after the package is placed on thecopper shape 102. There may, however, be portions of the copper shape102 that do not reside beneath the package after the package is placedon the copper shape 102. For example, a package may cover an area 104 ofthe copper shape 102 enclosed by the dotted line in FIG. 1A. In somedesigns, 90% or more of the copper shape 102 may reside beneath thepackage. In other designs less than 90% of the copper shape 102 mayreside beneath the package.

Before the package is placed on the pads 106 a, 106 b, a solder pastemay be placed on the pads 106 a, 106 b. The solder paste may temporarilyhold the solder balls of the BGA on the pads 106 a, 106 b. Oncecomponents have been placed on the PCB, the PCB may undergo a reflowprocess, during which the solder balls of the BGA become bonded to thepads 106 a, 106 b. During the reflow process, the PCB may undergowarpage. The heat dissipation of the copper shape 102 may contribute towarpage. Warpage may induce pad cratering under one or more of the pads106 a, 106 b. Pad cratering may be a separation between one or more ofthe pads 106 a, 106 b and a substrate of the PCB located beneath thepads 106 a, 106 b. Pad cratering may manifest as a crack beneath a pad.Pad cratering may reduce long-term reliability of a connection betweenthe package and the PCB. Pad cratering may be most severe wheredisplacement caused by warpage is increasing or decreasing rapidlybetween pads.

The copper shape 102 may include the voids 100. The voids 100 may reducewarpage associated with heat dissipation of the copper shape 102. Thevoids 100 may prevent pad cratering. The voids 100 may be introducedinto the copper shape 102 before a package is placed over the coppershape 102. The voids 100 may relieve stress during reflow and mitigatethe risk of solder ball cracks and pad cratering. The copper shape 102may satisfy applicable power-delivery requirements even with the voids100. The voids 100 may be indentations or holes in the copper pad 102.The voids 100 may be empty or filled with a non-copper substance. Thevoids 100 may not serve an insulating function. The voids 100 may not beused to connect a package to the PCB. The voids 100 may be covered by amask when solder paste is placed on the pads 106 a, 106 b.

The voids 100 may be generated through a chemical process, a mechanicalprocess, or other type of process. The voids 100 may be generated byremoving copper from the copper shape 102. For example, the voids 100may be generated by cutting the copper from the copper shape 102. Thevoids 100 may be generated by making indentations in the copper shape102. The voids 100 may be generated through use of chemical etching. Inthe alternative, the voids 100 may be created by not placing copper on asubstrate in certain areas of the copper shape 102.

The voids 100 may have a particular size at a surface of the coppershape 102. For example, the voids 100 may be 8 mil × 8 mil at thesurface of the copper shape. In the alternative, the voids 100 may besmaller in area or larger in area than 8 mil × 8 mil. In other designs,the voids 100 may have a specific minimum size or a specific maximumsize. For example, the voids 100 may all be at least 8 mil × 8 mil insize. As another example, the voids 100 may be at most 8 mil × 8 mil insize. Viewed looking down at the copper shape 102 from above, the voids100 may be shapes other than squares. For example, the voids 100 may belines, circles, triangles, quadrilaterals, rectangles, stars, or anytype of polygon. The voids 100 may be uniform in size and shape. In thealternative, the voids 100 may include etchings of different shapes andsizes. For example, some of the voids 100 may be 8 mil × 8 milindentations and some of the voids 100 may be 4 mil × 6 milindentations.

The voids 100 may go through the copper shape 102 to a substrate beneaththe copper shape 102. In the alternative, the voids 100 may not gothrough an entire depth of the copper shape 102. The voids 100 may notbe uniform throughout a depth of the voids 100. In other words, crosssections of the voids 100 at different depths of the copper shape 102may have different areas. In the alternative, the voids 100 may beuniform throughout the depth of the voids 100. The voids 100 do not gothrough every layer of the PCB. The voids 100 do not go through to aninner copper layer of the PCB. The voids 100 may be empty. In thealternative, the voids 100 may be filled with a non-copper substance.

In some designs, in areas of the copper shape 102 where the voids 100appear, the voids 100 may be routed in the copper shape 102 with uniformspacing. For example, there may be 8 mil gaps between the voids 100. Inother designs, in areas of the copper shape 102 where the voids 100appear, gaps between voids may be larger or smaller (such as 12 mil. or6 mil). In some designs, gaps may be measured in terms of an amount ofsurface area between two voids. For example, there may be 64 mil² ofsurface area between voids.

In some designs, in areas of the copper shape 102 where the voids 100appear, the voids 100 may be routed such that a certain number of voidsappear per unit area of the copper shape 102. For example, voids may berouted in a copper shape such that in areas of the copper shape wherethe voids appear, four voids appear per unit area (such as a 30 mil × 30mil area) of the copper shape. In other designs, in areas of the coppershape 102 where the voids 100 appear, the voids 100 may be routed suchthat a certain percentage of a surface area of the copper shape 102 isremoved by introducing the voids 100. For example, voids may be routedin a copper shape such that in areas of the copper shape where the voidsappear, thirty percent of a surface area of the copper is removed byintroducing the voids.

In some designs, in areas of a copper shape in which voids appear, thevoids may be introduced in the copper shape with non-uniform spacing ordensity. For example, in some designs, there may be gaps smaller than 8mil × 8 mil between some voids and gaps larger than 8 mil × 8 milbetween other voids. Gaps between the voids may be any shape or design.In some designs, a first number of voids may be placed per unit area ina first portion of a copper shape and, in a second portion of the coppershape, a second number (different from the first number) of voids may beplaced per unit area. In some designs, a first percentage of a firstsurface area of a copper shape may be removed using voids while a secondpercentage of a second surface area of the copper shape may be removedusing voids.

In areas of the copper shape 102 in which the voids 100 appear, thevoids 100 may be routed in one or more different patterns. A pattern maybe a particular arrangement of two or more voids that is repeated. Forexample, the voids 100 may be routed in a mesh pattern. A mesh patternmay be a pattern of generally uniform rows and columns. In otherdesigns, the voids may create a checkerboard pattern, a pattern ofdiagonal lines, a crossing pattern, or other pattern. In thealternative, in the areas of a copper shape in which voids appear, thevoids may not be routed with any particular pattern but may be generallyrandomly disbursed.

The voids 100 may be sufficient in number such that a pattern of thevoids 100 is arranged over a threshold percentage of a surface area ofthe copper shape 102. The surface area of the copper shape 102 may be anarea encompassed by an outer boundary of the copper shape 102. Forexample, a copper shape may include sufficient voids such that at least30% of a surface area of the copper shape includes a pattern of thevoids. The voids 100 may be sufficient in number such that a pattern ofthe voids 100 is arranged over a threshold percentage of a surface areaof the copper shape 102 that does not include the pads 106 a, 106 b. Forexample, a copper shape may include sufficient voids such that at least50% of an area of the copper shape that does not include pads includes apattern of the voids. The threshold percentage may depend on one or morefactors, such as a size of a copper shape, a type of package, a numberof solder balls in a ball grid array to be connected to the coppershape, and power needs of a package to be connected to the copper shape.For example, a first copper shape that will receive a first packagethicker than a second package to be received by a second copper shapemay have a higher threshold percentage of voids than the second coppershape.

The voids 100 may be arranged over a region of the copper shape 102having a threshold size. The size of the region may be an area of thecopper shape 102 where the voids 100 appear within a threshold distanceof each other. For example, the voids 100 may appear within 10 mil ofeach other in a 500 mil² area of the copper shape 102. As anotherexample, voids may be arranged over a region of the copper shape and theregion may be at least 200 mil² in size.

A specific number of the voids 100 may be introduced in the copper shape102. For example, 200 voids may be introduced in a copper shape. Asanother example, 20 voids may be introduced in a copper shape. In otherdesigns, a threshold number of the voids 100 may be introduced in thecopper shape 102. For example, at least 200 voids may be introduced in acopper shape. As another example, at least 10 voids may be introduced ina copper shape. The specific number or the threshold number may dependon one or more factors such as a size of a copper shape, a type ofpackage, a number of solder balls in a ball grid array to be connectedto the copper shape, and power needs of a package to be connected to thecopper shape. For example, a first copper shape with a size that islarger than a second copper shape may include more voids than the secondcopper shape.

The copper shape 102 may include one or more areas 132 that do notinclude the voids 100. The one or more areas 132 may not include thevoids 100 for purposes of power regulation or power flow. In otherdesigns, the one or more areas 132 may not include the voids 100 for adifferent purpose or for no purpose. In FIG. 1A, the one or more areas132 appear in a center of the copper shape 102. In other designs, theone or more areas 132 may be located in other portions of a coppershape. In FIG. 1A, the one or more areas 132 appear as a continuousarea. But in other designs, the one or more areas 132 may be separate byareas that include voids or pads. The location(s) and size of the one ormore areas 132 may be based on one or more factors such as a size of acopper shape, a type of package, a number of solder balls in a ball gridarray to be connected to the copper shape, power needs of a package tobe connected to the copper shape, a number of voids in the copper shape,and a density of voids in the copper shape. For example, one or moreareas of a copper shape may not include voids because the one or moreareas of the copper shape include a region of the copper shape that ismost proximate to pads connected directly to the copper shape ascompared to other regions of the copper shape.

FIG. 1B illustrates an example PCB 112. The PCB 112 may include multiplelayers. For example, the PCB 112 may include three or more conductivecopper layers. Surface sides of the PCB 112 may be conductive copperlayers. In between those layers and internal copper layers may beheat-protective insulation. Electrical connections between the layersare achieved with vias.

The PCB 112 may include multiple components attached to the PCB 112. Forexample, the PCB 112 may include a NAND 114 a, a NAND 114 b, a DRAM 116,a controller 118, and components 120 a-1. The NAND 114 a, the NAND 114b, the DRAM 116, the controller 118, and the components 120 a-1 may beconnected to the PCB 112 on a top layer of the PCB 112. The NAND 114 a,the NAND 114 b, the DRAM 116, the controller 118 may cover more surfacearea than the components 120 a-1. In other designs, a PCB may includemore or fewer components. In other designs, a PCB may include differentcomponents. The multiple components may be attached to the PCB 112 afterthe PCB 112 is manufactured. The PCB 112 may be designed specificallyfor certain components to be attached to the PCB 112. The components mayconnect with one or more other components on the PCB 112 throughmicrostrip or stripline connections.

The NAND 114 a may be a flash memory device or other non-volatilestorage technology. The NAND 114 a may include multiple die stacked oneon top of each other. For example, the NAND 114 a may be a DDP or a QDP.The NAND 114 a may be part of a package that includes a BGA. The BGA maybe attached to the package. The BGA may be an array of solder ballsarranged in a pattern that corresponds to a pattern of pads on the PCB112, such as the pads 106 a, 106 b. Placing the NAND 114 a on the PCB112 may have included placing the BGA on the pads of the PCB. During areflow process, the solder balls of the BGA may have bonded to the padsof the PCB 112.

The NAND 114 a may be placed over a copper shape, such as the coppershape 102. The NAND 114 a may overlap less then the entire copper shape.To mitigate warpage and protect against pad craters developing duringthe reflow process the copper shape underneath the NAND 114 a mayinclude voids, such as the voids 100.

The NAND 114 b may be a flash memory device or other non-volatilestorage technology. The NAND 114 b may be identical to the NAND 114 a.In the alternative, the NAND 114 b may be different from the NAND 114 a.For example, the NAND 114 a may be a DDP and the NAND 114 b may be aQDP. As another example, the NAND 114 a and the NAND 114 b may bothinclude BGAs but the BGAs may have different patterns of solder balls.In that case, the pattern of pads on the PCB 112 beneath the NAND 114 amay be different from the pattern of pads on the PCB 112 underneath theNAND 114 b. The NAND 114 b may be placed over a copper shape, such asthe copper shape 102. To mitigate warpage during the reflow process thecopper shape underneath the NAND 114 a may include voids, such as thevoids 100.

The DRAM 116 may be dynamic random-access memory or another volatilememory technology. The DRAM 116 may be part of a package that includes aBGA. The BGA may be an array of solder balls arranged in a pattern thatcorresponds to a pattern of pads on the PCB 112. During a reflowprocess, the solder balls of the BGA may bond to the pads of the PCB112. The DRAM 116 may be placed over a copper shape. To mitigate warpageand resulting pad craters during the reflow process the copper shapeunderneath the DRAM 116 may include voids.

The controller 118 may be a device that interfaces with components ofthe PCB 112. The controller 118 may be part of a package that includes aBGA. The BGA may be an array of solder balls arranged in a pattern thatcorresponds to a pattern of pads on the PCB 112. During a reflowprocess, the solder balls of the BGA may bond to the pads of the PCB112. The controller 118 may be placed over a copper shape. To mitigatewarpage and resulting pad craters during the reflow process the coppershape underneath the controller 118 may include voids.

The components 120 a-1 may be electronic components that do not includea BGA. The PCB 112 may include pads corresponding to the components 120a-1. Before placing the components 120 a-1 on the PCB 112, solder pastemay be placed on the pads corresponding to the components 120 a-1. Thecomponents 120 a-1 may be placed on the solder paste. During the reflowprocess, the solder paste may bond the components 120 a-1 to the PCB112. The pads corresponding to the components 120 a-1 may be smallerthan the copper shapes underneath the NAND 114 a, the NAND 114 b, theDRAM 116, and the controller 118.

Each of the components 120 a-1 may cover less surface area of the PCBthan the NAND 114 a, the NAND 114 b, the DRAM 116, and the controller118. The surface area of the copper shapes underneath the NAND 114 a,the NAND 114 b, the DRAM 116, and the controller 118 may each cover agreater area of the PCB 112 than any of the components 120 a-1. Forexample, the copper shape underneath the NAND 114 a may cover a greaterarea of the PCB 112 than the component 120 c.

FIG. 2 illustrates an example of a copper shape 202 that does notinclude voids. The copper shape 202 may be identical to the copper shape102 except that the copper shape 102 includes the voids 100 and thecopper shape 202 does not include voids. For example, the copper shape202 may include pads 206 a and pads 206 b. The pads 206 a and the pads206 b may be arranged in a pattern identical to the pattern of the pads106 a and the pads 106 b. The pads 206 a may include pad 208 a. The pads206 b may include pad 208 b. The pad 208 a and the pad 208 b may be partof the copper shape 202 or connect directly to the copper shape 202. Thecopper shape 202 may include protrusions 210 a, 210 b. A package maycover an area 204 of the copper shape 202 shown in FIG. 2 by a dottedline.

The copper shape 202 may experience, during a reflow process, greaterwarpage than the copper shape 102 experiences. The copper shape 202 mayexperience, during a reflow process, a greater difference indisplacement across a diagonal of the copper shape 202 than the coppershape 102 experiences. An example of these differences is illustrated inFIG. 4 . These differences may contribute to the copper shape 202experiencing pad cratering and the copper shape 102 not experiencing padcratering. As a result, the long-term reliability of connections of thecopper shape 102 may be better than the copper shape 202.

FIG. 3A illustrates a bottom side of a package 314 and a top side of acopper shape 302 that may reside on a top side of a PCB. The package 314may include an integrated circuit device, such as a NAND. The package314 may include multiple die.

The bottom side of the package 314 may include a BGA 322. The BGA 322may include solder balls 322 a and solder balls 322 b. The solder balls322 a, 322 b may be balls of solder that connect the package 314 to thePCB. The solder balls 322 b may include solder ball 322 b-1 and solderball 322 b-2.

The copper shape 302 may be a continuous sheet of copper except for apad array 306. The pad array 306 may include pads 306 a and pads 306 b.In FIG. 3A, the pads 306 a, 306 b are shown to have a square shape forease of visually distinguishing them in FIG. 3A from the solder balls322 a, 322 b. Pads may, however, take any shape, including a generallycircular shape as shown in FIG. 1A. The pads 306 a may include pad 308a. The pads 306 b may include pad 308 b and pad 306 b-1. Other than thepads 308 a, 308 b, the pads 306 a, 306 b may be areas of exposed coppersurrounded by anti-pad. The anti-pad may insulate the exposed copperfrom the copper shape 302. The pad 306 b-1 may be a pad surrounded byanti-pad. The pads 308 a, 308 b may not have anti-pad around them.Instead, the pads 308 a, 308 b may be a part of or connect directly tothe copper shape 302. The pads 308 a, 308 b may be designed to providepower to the package 314.

The solder balls 322 a, 322 b of the BGA 322 may be arranged in aparticular pattern. The particular pattern may match a pattern of thepads 306 a, 306 b of the pad array 306. The pads 306 a, 306 b may beareas where solder paste is placed before the package 314 is attached tothe PCB. Solder paste may not be placed on any area within a surfacearea of the copper shape 302 except the pads 306 a, 306 b. After thesolder paste is placed on the pads 306 a, 306 b, the solder balls 322 a,322 b of the BGA 322 may be placed on the pads 306 a, 306 b of the padarray 306. During the reflow process, the solder balls 322 a, 322 b maybond to the pads 306 a, 306 b.

The copper shape 302 may include voids 300. The voids 300 may beindentations in the copper shape 302 where no copper is present. Thecopper shape 302 may also include one or more areas 332 where the voids300 are not introduced. The voids 300 may be numerous enough to reducewarpage experienced by the PCB during the reflow process. The voids 300may be numerous enough to reduce the warpage experienced by the PCB orthe copper shape 302 during the reflow process by a certain factor orpercentage as compared to the warpage experienced by a PCB or a coppershape that does not include voids. The voids 300 may be sufficientlynumerous to reduce a difference in displacement experienced during areflow process between a middle of the copper shape 302 and an edge ofthe copper shape 302 as compared to a copper shape that does not includevoids.

FIG. 3B illustrates a cross-section side view of the package 314 and aPCB 312 on which the copper shape 302 resides. The cross-section sideview may be along a diagonal of the copper shape 302. The BGA 322 andthe pad array 306 may align. When the BGA 322 is placed on the pad array306, the solder ball 322 b-1 may contact the pad 306 b-1. The solderballs 322 b of the BGA 322 may not be uniform in height. For example,the solder ball 322 b-2 may be taller than the solder ball 322 b-1. Thisdifference in height may contribute to the solder ball 322 b-2experiencing different forces and displacement during the reflow processthan the solder ball 322 b-1. The difference in forces and displacementexperienced by the solder ball 322 b-2 and the solder ball 322 b-1 maycontribute to solder joints in between the solder ball 322 b-2 and thesolder ball 322 b-1 being susceptible to solder cratering. The voids 300in the copper shape 302 may reduce the difference in the forces anddisplacement experienced by the solder ball 322 b-2 and the solder ball322 b-1 and reduce a likelihood that any pads in the pad array 306experience pad cratering.

FIG. 3C illustrates a close-up cross-section side view of the solderball 322 b-1 and the pad 306 b-1 when the package 314 is placed on thePCB 312. The solder ball 322 b-1 may attach to the package 314 at a BGApad 328. The BGA pad 328 may be surrounded by solder mask 326 a, 326 b.The pad 306 b-1 may attach to the PCB 312. The pad 306 b-1 may connectto one or more inner layers of the PCB 312. Solder mask 324 a, 324 b mayact as anti-pad around the pad 306 b-1. Solder paste may be placed onthe pad 306 b-1 before the solder ball 322 b-1 is placed on the pad 306b-1. During the reflow process, the solder ball 322 b-1 may bond to thepad 306 b-1. The solder ball 322 b-1 may also experience displacementduring the reflow process due to warpage of the PCB 312 and otherforces. Any of the solder joints formed by any of the solder balls 322a, 322 b and the pads 306 a, 306 b (other than the pads 308 a, 308 b)may take the form shown in FIG. 3C.

Under certain conditions, a pad (such as the pad 306 b-1) may experiencepad cratering and pull away from the PCB 312, resulting in a crackbetween the pad and the PCB 312. Pad cratering may result in lowerreliability of the PCB 312. The voids 300 in the copper shape 302 mayreduce the likelihood that any of the pads 306 a, 306 b experience padcratering during the reflow process. The voids 300 in the copper shape302 may prevent the pads 306 a, 306 b from experiencing pad crateringduring the reflow process. The voids 300 may reduce warpage experiencedby the PCB 312 and the copper shape 302 during the reflow process. Thevoids 300 may reduce a difference in displacement experienced by the pad306 b-1 and a pad corresponding to the solder ball 322 b-2.

FIG. 4 shows graph 436 a and graph 436 b. The graph 436 a may representdisplacement experienced by points along a diagonal line from a bottomleft corner of the copper shape 102 to a top right corner of the coppershape 102 shown in FIG. 1A. The displacement may be measured at aparticular temperature (such as 150° F.) during a simulation of a reflowprocess. The graph 436 b may represent displacement experienced bypoints along a diagonal line from a bottom left corner of the coppershape 202 to a top right corner of the copper shape 202 shown in FIG. 2. The displacement may be measured at a particular temperature (such as150° F.) during a simulation of a reflow process. The graph 436 a andthe graph 436 b illustrate that the copper shape 102, which includes thevoids 100, experiences less change in displacement across the coppershape 102 than the copper shape 202, which does not include voids. Forexample, between pixel 20 and pixel 30 in the graph 436 a, the change indisplacement may be less than five microns. In the graph 436 b, incontrast, the change in displacement between pixel 20 and pixel 30 isclose to 10 microns. The rate of change of displacement across thecopper shape 102 is also less than the copper shape 202. Thesedifferences may result at least in part from the voids 100 in the coppershape 102. These differences may result in the copper shape 102 notexperiencing pad cratering while the copper shape 202 does experiencepad cratering.

FIG. 5A illustrates an example copper shape 502. The copper shape 502may include pad area 530 a and pad area 530 b. The pad areas 530 a, 530b may be regions of the copper shape 502 where pads are located. The padareas 530 a, 530 b may include areas between the pads. The pad areas 530a, 530 b may be regions of the copper shape 502 in which pads are withina threshold distance of each other. In FIG. 5A, shows two pad areas, thepad area 530 a and the pad area 530 b. In other designs, there may befewer pad areas or more pad areas. In other designs, pad areas may haveshapes different from the shape of the pad areas 530 a, 530 b

The copper shape 502 may include region 532, which may be an area of thecopper shape 502 other than the pad areas 530 a, 530 b. The region 532may be covered by a mask when solder paste is placed on pads in the padareas 530 a, 530 b. Portions of the pad areas 530 a, 530 b that are notpads may also be covered by the mask.

In some designs, voids are located only in the region 532 and not thepad areas 530 a, 530 b. Voids may be placed in some of the region 532.In other designs, voids may be placed throughout the region 532. Voidsmay be placed with uniform density in some of or throughout the region532. In other designs, voids may be routed with non-uniform density.Voids may be routed in a uniform pattern in some of or throughout theregion 532. Voids may be routed in multiple patterns in some of orthroughout the region 532. Voids may be routed in one or more densitiesin some of or throughout the region 532. Voids may be routed in athreshold percentage of the region 532. A threshold number of voids maybe routed in the region 532.

FIG. 5B illustrates example patterns 534 of voids that may be placed inthe region 532. The example patterns 534 include a checkboard pattern534 a, a pattern 534 b, a diagonal mesh pattern 534 c, a line pattern534 d, a zig-zag pattern 534 e, a crossing pattern 534 f, a circlepattern 534 g, and a mesh pattern 534 h.

FIG. 6 illustrates an example method 600 for mitigating pad cratering.The method 600 may include fabricating 602 a printed circuit board,wherein the printed circuit board includes a copper shape on a top layerof the printed circuit board. An outer edge of the copper shape mayenclose an area larger than an area covered by an integrated circuitpackage to be placed over the copper shape.

The method 600 may include fabricating 604 an array of pads within anarea of the copper shape. The array of pads may include one or more padssurrounded by anti-pad.

The method 600 may include introducing 606 voids in the copper shape.Introducing 606 the voids in the copper shape may include introducing asufficient number of voids in the copper shape to reduce warpageexperienced by the printed circuit board or the copper shape during areflow process by a threshold amount or percentage. For example, asufficient number of voids may be added to the copper shape to reducewarpage experienced by the copper shape by 10%, 20%, 50%, or more.Introducing 606 the voids in the copper shape may include introducing asufficient number of voids in the copper shape to reduce a difference indisplacement across the copper shape.

The method 600 may include placing 608 a package on the printed circuitboard, wherein the package overlaps at least a portion of the coppershape and includes a ball grid array. The ball grid array may comprise apattern of solder balls that mirrors the array of pads within the areaof the copper shape. Placing 608 the package on the printed circuitboard may include placing solder paste on the array of pads and placingthe ball grid array on the array of pads. The package may be a multi-diepackage. The package may be a NAND.

The method 600 may include bonding 610 the package to the printedcircuit board. Bonding 610 the package to the printed circuit board mayinclude placing the printed circuit board in a reflow oven and exposingthe printed circuit board to a solder reflow process. The resultingprinted circuit board may pass a dye and pry test, even where thepackage is a multi-die package.

The steps, operations, and/or actions of the methods described hereinmay be interchanged with one another without departing from the scope ofthe claims. In other words, unless a specific order of steps,operations, and/or actions is required for proper functioning of themethod that is being described, the order and/or use of specific steps,operations, and/or actions may be modified without departing from thescope of the claims.

The terms “comprising,” “including,” and “having” are intended to beinclusive and mean that there can be additional elements other than thelisted elements. Additionally, it should be understood that referencesto “one embodiment” or “an embodiment” of the present disclosure are notintended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. For example, anyelement or feature described in relation to an embodiment herein may becombinable with any element or feature of any other embodiment describedherein, where compatible.

The present disclosure may be embodied in other specific forms withoutdeparting from its spirit or characteristics. The described embodimentsare to be considered as illustrative and not restrictive. The scope ofthe disclosure is, therefore, indicated by the appended claims ratherthan by the foregoing description. Changes that come within the meaningand range of equivalency of the claims are to be embraced within theirscope.

1. A system for mitigating warpage of a printed circuit board, thesystem comprising: the printed circuit board, the printed circuit boardcomprising: an outer copper layer; a copper shape within the outercopper layer, wherein the copper shape has an outer edge that enclosesan area and the area of the copper shape includes voids; and an array ofpads within the area enclosed by the copper shape; and a package, thepackage comprising: at least one silicon die; and a ball grid array, theball grid array comprising solder balls configured to connect with thearray of pads.
 2. The system of claim 1, wherein the voids aresufficient in number to reduce a difference in displacement experiencedduring a reflow process between a middle of the copper shape and an edgeof the copper shape.
 3. The system of claim 1, wherein the voids arearranged in a mesh pattern and the mesh pattern is arranged over 10% ofthe area enclosed by the copper shape.
 4. The system of claim 3, whereinthe voids are 8 mil x 8 mil.
 5. The system of claim 1, wherein the atleast one silicon die comprises two or more silicon die.
 6. The systemof claim 5, wherein the solder balls are not uniform in height.
 7. Thesystem of claim 1, wherein the printed circuit board includes three ormore layers of copper and the voids do not penetrate to a layer ofcopper below the outer copper layer.
 8. The system of claim 1, wherein aregion of the area enclosed by the copper shape does not include thevoids and wherein the array of pads comprises one or more pads connecteddirectly to the copper shape, the copper shape provides power to thepackage, and the region is more proximate to the one or more padsconnected directly to the copper shape than at least one other region ofthe copper shape.
 9. The system of claim 1, wherein the copper shapeincludes 30 or more voids.
 10. A printed circuit board comprising: anouter copper layer; a copper shape within the outer copper layer,wherein the copper shape includes at least 10 voids; and an array ofpads arranged within an area of the copper shape, the array of padsconfigured to receive a ball grid array attached to a package.
 11. Theprinted circuit board of claim 10, wherein the voids are sufficient innumber to reduce warpage experienced by the copper shape during a reflowprocess.
 12. The printed circuit board of claim 10, wherein one or moreof the voids are at least 8 mil x 8 mil.
 13. The printed circuit boardof claim 10, wherein the printed circuit board includes three or morelayers of copper and the voids do not penetrate to a layer of copperbelow the outer copper layer.
 14. The printed circuit board of claim 10,wherein the copper shape includes voids arranged over at least fiftypercent of a portion of the copper shape that does not include the areawhere the array of pads is arranged.
 15. A method for mitigating warpagein a printed circuit board, the method comprising: fabricating a coppershape on a top layer of the printed circuit board; fabricating an arrayof pads within an area of the copper shape; and introducing voids in thecopper shape over a region outside the area, wherein the regioncomprises at least ten percent of the copper shape.